Building block



FIG. 1 is a top perspective view of the building block of my ornamental design;

FIG. 2 is a bottom perspective view thereof;

FIG. 3 is a side elevational view thereof;

FIG. 4 is a top plan view thereof; and,

FIG. 5 is a bottom plan view thereof.

The broken lines represent portions of the article that form no part of the claim. 

The ornamental design for a building block, as shown and described. 